1. Field of the Invention
This invention relates to a hydrofoil support or box for use in a paper making machine of the type wherein hydrofoil blades are positioned beneath a forming medium and extended in the cross machine direction relative to the forming medium for draining water through the forming medium from a paper web being formed on the forming medium and for forming the paper web. This invention also relates to a hydrofoil blade and to a method using hydrofoil blades for dewatering and forming a paper web.
2. Description of the Prior Art
In the typical Foundrinier papermaking machine, an aqueous suspension of fibers, called the "stock" is flowed from a headbox onto a traveling Fourdrinier wire or medium, generally a woven belt of wire and/or synthetic material, to form a continuous sheet of paper or paper-like material. In this connection, the expression "paper or paper-like material"is used in a broad or generic sense and is intended to include such items as paper, kraft, board, pulp sheets and non-woven sheet-like structures. As the stock travels along on the Fourdrinier wire, formation of a paper web occurs, as much of the water content of the stock is removed by draining. Water removal is enhanced by the use of such well-known devices as hydrofoil blades, table rolls and/or suction devices. This invention relates to hydrofoil blades.
The hydrofoil blades used in papermaking perform two functions. The first function is to create a vacuum pulse over the downward inclined face of the hydrofoil blade. This pulse removes a portion of the white water from the lower side of the stock or three-dimensional fiber suspension which lays upon the forming medium and causes some of the fibers to be laid down and formed into a web. The amount of such water removal and web formation over a given hydrofoil blade is small, and therefore a considerable number of blades is required to form all of the fibers in a stock suspension into a two dimensional web. For example, the use of ten to fifty hydrofoil blades is not uncommon. In other words, the sheet forming process is a step-by-step filtration process as the forming medium travels over the hydrofoil blades, with some of the fibers in the lower portion of the suspension over the partially-formed web being added to the web at each successive foil blade. The average net change in fiber concentration or consistency of this process ranges from the headbox consistency, which is usually about 0.4% to about 1%, up to about 2.5%.
The second function of a hydrofoil blade is to maintain the fibers which are still in suspension throughout the forming process in an as-well-as dispersed condition as possible; i.e., in a deflocculated condition. This function is extremely important as fibers in the 0.5-2.5% consistency range have a strong tendency to flocculate into clumps on their own in a matter of milliseconds once the fiber dispersive forces have decayed. This flocculation causes the final paper to be highly non-uniform or flocculated in appearance.
The fiber dispersive function of hydrofoil blades is caused primarily by the decay of the dewatering vacuum pulse which imparts a momentary upward force or pulse into the stock. This pulse creates random small scale flows, i.e., turbulence, in the stock above the partially-formed web. The greater the angle of the downward inclined part of the hydrofoil blade, the greater this deflocculating pulse or turbulence. The speed of travel of the suspension over the blade is also a factor in determining the intensity of this pulse. Thus, at high machine speeds, the size of the hydrofoil blade angle which can be used is limited lest the vacuum be so large that the pulse created will throw some of the stock upward into the air. This phenomenon known as "stock jump" can readily damage the uniformity of the sheet.
One aspect of hydrofoil blade dewatering overlooked in the past is that when the vacuum pulse created by the inclined angle of the hydrofoil blade decays back to atmospheric pressure, the decay is somewhat of an unstable phenomenon. This is because the hydrofoil blade generally discharges the water removed from the suspension directly into the atmosphere. In other words, the decay of the vacuum pulse occurs virtually instantaneously at the point where the gap between forming medium and hydrofoil blade becomes too large to support a continuous column of water. The location of this point is extremely sensitive to all of the forces and resistances affecting the dewatering process as evidenced by the highly variable amount of water removed from the suspension across the width of foil blades. This variability can be readily observed on any paper making machine. The water removed from the suspension by any one hydrofoil blade is largely carried along the underside of the forming medium to the next blade whose leading edge skives the water off the underside of the forming medium. The amount of skived water varies very considerably from point to point across the width of a machine at most hydrofoil blade positions.
The variability of dewatering in the cross machine direction of the hydrofoil blades is further exacerbuted by the slight non-uniformity of wear of the high density ultra-high molecular weight polyethylene of which most hydrofoil blades are made, as well as by the non-uniform build-up of fibrous material on the leading edge of many blades. These problems of polyethylene hydrofoil blades have led to the development of a variety of ceramic blades which are much more wear resistant. While ceramic blades hold their shape much better than polyethylene blades, they are extremely fragile, prone to damage, and relatively expensive. Since such blades are difficult to handle, once a Fourdrinier table has been laid out, papermakers are loathe to alter blades. Thus, the use of ceramic hydrofoil blades is limited.
The cross machine direction variability of dewatering of hydrofoil blades is one, if not the primary source of the non-uniformity of the "dry line", i.e., the line across the Fourdrinier where air is first introduced into the wet web over the vacuum foils or suction boxes. This variability ultimately leads to the cross-direction variation in the moisture content of the finished paper, one of the most critical problems facing the paper industry.
Another problem created by this turbulence generating pulse of hydrofoil blades is that it loosens up the structure of the partially formed web and allows for the finer fibers as well as the filler particles to be washed out of the web. Thus, the stronger the vacuum pulse of a foil blade, the lower the fines and filler retention in the lower part of the web. This top-to-bottom side variation of fines and fillers in a sheet is a major source of many paper application problems well-known to those skilled in the art.
Turning now to another aspect of hydrofoil blade applications and problems, a new forming strategy has been evolved in which it is desirable to minimize or even totally eliminate the turbulence on the Fourdrinier wire generated by the hydrofoil blades. This new approach employs formation showers which create stock ridges which periodically collapse and reform on their own down the wire. The collapse and regeneration of these ridges creates a cross machine direction shear which deflocculates fibers in much the same way as the cross machine direction shear generated by the shake of slower papermaking machines. The advantage of these ridges over shakes is that the ridges can be employed and are effective at any machine speed including relatively high ones, whereas the effective application of the shake is limited to machine speeds below 300-400 m/min. The stock ridges formed by formation showers are extremely fragile fluid structures which are easily destroyed by the turbulence generated by hydrofoil blades.
It is clear from all of the foregoing that there is a need for hydrofoil blades wherein the amount of water removed from the suspension across each blade width; that is, the dewatering in the cross machine direction, is controlled by stabilizing the vacuum decay zone of each blade. In addition, the absence of a strong pressure pulse following dewatering is desirable in order to obtain a higher and more uniform fines and filler retention. Further, there is a need in some applications for a dewatering hydrofoil blade system which does not generate turbulence.